1. Field of the Invention
The present invention relates to a slide rail device used in a vehicle seat, an automotive seat or the like, which is adapted to adjust the forward and rearward positions of the seat, and in particular, relates to a slide rail device of the type having a seat belt provided on its upper rail and designed to avoid the separation of upper rail from lower rail in a sudden deceleration case due to a collision accident or the like.
2. Description of the Prior Art
Heitherto, there have been known various kinds of slide rail devices for vehicle seat, and among them, a type of the device has been in use, which has a seat belt anchored thereto in order to disperse a pulling force exerted on the seat belt to the floor of the vehicle and further so desinged to prevent the disengagement of its upper rail from its lower rail for safety purpose against a sudden deceleration case when a collision accident or the like occurs.
In this sort of slide rail device, one end portion of the seat belt is fixedly connected to its upper rail in most cases. The seat belt restrains an occupant who sits on the seat provided above the upper rail, and the restraint force of the seat belt is supported on the upper rail.
Various examples of such slide rail device exist as known, for example, from the Japanese Utility Model Publication No. 57-24594, or the U.S. Pat. No. 4,204,658.
Those prior art devices are basically similar to such construction as shown in FIG. 1 of the accompanying drawings. Hereinafter, the structural advantages and disadvantages of the prior art will be discussed with reference to the model in FIG. 1. Namely, in the prior art, a plurality of equidistant apertures (102) are formed in the shoulder portion (130) of an upper rail (114), and a plurality of engagement tooth (104), which are equidistant from one another corresponding to each of the apertures (102), are formed at the end of an upper curved flange portion (128) of a lower rail (112). The engagement tooth (104) are respectively disposed above each of the apertures (102). With this structure, in the case of a sudden deceleration due to a collision accident, the upper rail (114) is applied a pulling force (F) (See FIG. 2) which tends to raise the upper rail (114), and thus, as the upper raill (114) is moved upwardly, the tooth (104) of the lower rail (112) are inserted into and engaged in the respective apertures (102) of the upper rail (114), as shown in FIG. 2. Accordingly, the upper rail (114) are prevented from its separation from the lower rail (112).
However, the pulling force (F) being applied is, in fact, oriented obliquely in a directin inwardly of the slide rail device, as shown in FIG. 2. Such pulling force (F) is in turn exerted upon the upper curved flange portion (128) because of the full engagement of the tooth (104) in the respective apertures (102). Then, the upper curved flange portion (128), which is formed in a cantilever fashion, is easily bent upwards relative to a base point (a), with the result that the tooth (102) are raised at the same time, as illustrated in the phantom line in FIG. 2. This is, in the dynamic sense, attributed to a bending moment caused by the pulling force (F), and in particular, a greater amount of such bending moment is caused in the series of the tooth (102) disposed outwardly of the device (i.e. the one disposed on the left side as viewed in FIG. 2). As a result, there is a great likelihood of those outwardly disposed series of tooth (102) being disengaged out of their corresponding apertures (102) one by one. Thus, according to the present prior art, it is not possible to prevent the upper rail (114) against its separation from the lower rail (112), completely.
Of course, the foregoing problem can be solved by forming the upper and lower rails to have more thickness for increasing their rigidity against deformation. But, such formation of the upper and lower rails results in a more increased dimensions of the slide device as a whole, requiring a large sufficient room under the seat, and also results in a more increased weight of the device, which is not economical in costs for assemblage thereof and disadvantageous in providing a light weight of the seat.
Further, in contrast to the above-mentioned prior art, there has been such slide rail device as disclosed in the Japanese Laid-Open Utility Model Publication No. 49-106021. According to this another prior art, the upper rail is at its lower portion formed with a substantially U-shaped curved portion and the lower rail is also formed with a likewise curved portion in such a manner that the curved portion of the upper rail encloses that of the lower rail. Nevertheless, it is higly possible for the upper rail to be separated from the lower one, because when a great upward pulling force is exerted on the upper rail, the curved portion of the upper rail is easily opened, resulting in the upper rail being slipped off from the curved portion of the lower rail and separated therefrom.